The Adriatic Sea has a very marked annual change of the surface temperature. The average annual temperature is 11°C. During the winter, the sea is the coldest and the surface temperature is about 7°C; very seldom, it can drop below that too. In the spring, the sea becomes warmer, and the surface temperature rises to 18°C. In the summer the surface of the sea reaches a very high temperature, of up to 22 to 25°C, and in the southern Adriatic and Istria up to 27°C. In the Adriatic, thermoclines, i.e. parts of the water column of the same temperature, are very well distinguished. The thermocline is most evident during the summer, and, in the winter, the isothermal process arises, i.e. equaling of the temperature throughout the water column. In the summer, we can notice the first thermocline at the depth of 3 to 5 metres, the next one is at about 12 metres, and yet another one at 18 metres, while below 30 metres the temperature is mostly constant throughout the year.